Rasmussen's Liberal-Conservative coalition and their allies, who came to power in 2001, won 54% of the four million votes against 46% polled by the centre-left opposition.
"The political shift which began in 2001 is reaffirmed. The Social Democrats and left wing have lost further and a stronger government can continue its work," said Rasmussen, the first Liberal prime minister ever to be re-elected.
He had called a snap election three weeks ago to capitalise on what he termed a "fairy tale" economic and social scenario.
Immigration dominated the brief campaign. Laws passed in 2002 making it harder to bring foreign spouses into Denmark and to qualify for asylum have cut immigration sharply.
Immigrants account for just 2% of the electorate.
Rasmussen, who sent troops to Iraq to help in the US-led war, had warned a victory for the main opposition Social Democrats would "loosen up" asylum policies.
"I am sad that Danes will now have to live with a Liberal government propped up by the Danish People's Party," said the defeated Social Democrat leader Mogens Lykketoft, referring to the ruling coalition's anti-immigrant allies.
However, the opposition Social Liberal Party almost doubled its support in what party leader Marianne Jelved called a "big victory" for its calls for softer asylum laws.
The Danish government says its asylum laws are "firm but fair".
Immigration Minister Bertel Haarder said it was "extremely difficult to integrate newcomers into the labour market if they have no qualifications in Danish language".